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The existing method for contactless measurement of the photoconductivity decay time is limited in terms of sample selection according to the injection level or doping density. To solve this problem and improve the measurement sensitivity, we developed a quasi-optical photoconductivity decay (QO-PCD) technique based on millimeter- and terahertz-wave technology. A semi-insulating silicon (Si) wafer was used in a proof-of-concept experiment with the proposed QO-PCD system to find the initial excess carrier density and carrier lifetime based on the Drude-Zener model with a single decay function. The initial excess carrier density and carrier lifetime were measured to be 1.5 × 10cmand 30.6 μs, respectively, in semi-insulating Si wafer (460 μm thickness). A 2D areal measurement of the decay time of the Si wafer was experimentally obtained. The proposed QO-PCD technique can provide more reliable and sensitive carrier lifetime measurement data for semiconductor wafers, which may impact the fields of photovoltaic solar cells and power electronics.